debugging asserts

Chapter09/Cargo.toml

@@ -79,3 +79,8 @@ path = "debugging_heartbeat.rs"
 [[bin]]
 name = "debugging_buggy_worker"
 path = "debugging_buggy_worker.rs"
+
+[[bin]]
+name = "debugging_assert"
+path = "debugging_assert.rs"
+

Chapter09/debugging_assert.rs

@@ -0,0 +1,37 @@
+use std::io;
+
+fn debug_precondition(n: u64) -> u64 {
+   debug_assert!(n < 100);
+   n * n
+}
+
+fn debug_postcondition(n: u64) -> u64 {
+   let r = n * n;
+   debug_assert!(r > 10);
+   r
+}
+
+fn runtime_precondition(n: u64) -> Result<u64,()> {
+   if !(n<100) { return Err(()) };
+   Ok(n * n)
+}
+
+fn runtime_postcondition(n: u64) -> Result<u64,()> {
+   let r = n * n;
+   if !(r>10) { return Err(()) };
+   Ok(r)
+}
+
+fn main() {
+   //inward facing code should assert expectations
+   debug_precondition(5);
+   debug_postcondition(5);
+
+   //outward facing code should handle errors
+   let mut s = String::new();
+   println!("Please input a positive integer greater or equal to 4:");
+   io::stdin().read_line(&mut s).expect("error reading input");
+   let i = s.trim().parse::<u64>().expect("error parsing input as integer");
+   runtime_precondition(i).expect("runtime precondition violated");
+   runtime_postcondition(i).expect("runtime postcondition violated");
+}